1. Field of the Invention
This invention relates to a power supply system, and particularly to a structure suitable for multiple ranges of input voltage, which integrates a rectifier/filter""s circuitry and a converter""s circuitry with an inverter to reduce volume and increase power efficiency.
2. Description of the Related Art
Household power supply typically ranges from 90-132 Vac and 180-264 Vac. However, in current LCD monitors, a DC source with lower voltage than the power supply is used to power all circuits, e.g. the video control circuit, except that the discharge lamp for illumination is powered by an AC source with higher voltage than the power supply. For example, a mono-lamp notebook requires about 7-21 Vdc while a multi-lamp LCD monitor requires the rated voltage about 12 or 15 Vdc. Also, the monitor requires more than 1000 Vac to drive a cold cathode fluorescent lamp (CCFL) for illumination. Therefore, to meet the above requirements, a typical power supply system, as shown in FIG. 1, must include an AC source input from a socket passing through a rectifier/filter 11, a fly-back converter 12, a DC-AC inverter 13 and a buck regulator 14 to provide the lamp(s) with AC power and other elements of the display system with DC power. As such, the typical power supply system must convert between AC and DC in too many stages, which causes inconvenience and inefficiency. In current products, the rectifier/filter 11 and the fly-back converter 12 are combined together to form an additional adapter 10, which is further connected to the inverter 13 and the buck regulator 14 via additional connectors and cables (not shown). Accordingly, such a product carries power efficiency only to about 70%, high production costs and larger dimensions.
Therefore, an object of the invention is to provide a power supply with reduced dimensions and increased power efficiency without the need of an additional adapter. The power supply for powering a system having a lamp includes a rectifier/filter, a DC-DC converter and a DC-AC inverter. The rectifier/filter has an input terminal for inputting AC voltage in order to convert the input AC voltage into DC voltage. The DC-DC converter and DC-AC inverter are parallel to each other with one end concurrently connected to the rectifier/filter""s output and the other end respectively outputting the power required by the system. As such, DC-DC converter reduces the converted DC voltage to the lower DC voltages to power all circuits except for the lamp, and DC-AC inverter converts the converted DC voltage to a higher AC voltage output to drive the lamp.
Accordingly, the inventive power supply can directly integrate the rectifier/filter, converter and inverter to increase power efficiency. Moreover, components with lower rated power can be used and the power supply can be arranged on a single circuit board. Therefore, the volume is reduced and the component cost and assembling cost are both lowered.
A further object of the invention is to provide an inverter for driving a discharge lamp, the inverter including: two switches, a driver for driving the two switches alternately turned on, a transformer, a sampling circuit for obtaining the current value through the lamp and outputting a feedback signal, a PWM control circuit for controlling the duty cycle of the driver according to the feedback signal, a voltage detection circuit for outputting a control signal according to the DC voltage received by the inverter, and an impedance adjustment circuit for adjusting the equivalent impedance value of the inverter according to the control signal.
Accordingly, the inventive inverter can change the frequency-to-impedance curves through the impedance adjustment circuit""s adjustment when the input voltage is higher. Therefore, the operating frequency of the inverter will not change remarkably with the increasing input voltage. The invention thus ensures a longer lifespan of the lamp and avoids the temperature-increasing problem due to the skin effect on the wires during high-frequency operation to thereby reduce the converting loss.